With gel electrophoresis for studying chromosomal proteins, we shall attempt to identify initiator factors important for DNA synthesis. If successful, the effects of ionizing radiation on these factors will be studied. As a part of this study, the effects of radiation on transport of TdR, UdR, CdR, and leucine into the cell will be studied. Also, the effects of x-irradiation will be compared with effects of drugs such as puromycin, CHM, and FUdR, on the initiation and rate of DNA synthesis in individual replicons. A further definition of the x-ray-induced G2 block in terms of effects on specific chromosomal proteins and possible ultrastructural changes will be continued. Variations in the x-ray and heat sensitivity during the cycle for induction of mutations will be compared with variations observed for cell killing and chromosomal aberrations.